Blade-Resolved CFD Simulations of a Periodic Array of NREL 5 MW Rotors with and without Towers

A fully resolved (FR) NREL 5 MW turbine model is employed in two unsteady Reynolds-averaged Navier–Stokes (URANS) simulations (one with and one without the turbine tower) of a periodic atmospheric boundary layer (ABL) to study the performance of an infinitely large wind farm. The results show that the power reduction due to the tower drag is about 5% under the assumption that the driving force of the ABL is unchanged. Two additional simulations using an actuator disc (AD) model are also conducted. The AD and FR results show nearly identical tower-induced reductions of the wind speed above the wind farm, supporting the argument that the AD model is sufficient to predict the wind farm blockage effect. We also investigate the feasibility of performing delayed-detached-eddy simulations (DDES) using the same FR turbine model and periodic domain setup. The results show complex turbulent flow characteristics within the farm, such as the interaction of large-scale hairpin-like vortices with smaller-scale blade-tip vortices. The computational cost of the DDES required for a given number of rotor revolutions is found to be similar to the corresponding URANS simulation, but the sampling period required to obtain meaningful time-averaged results seems much longer due to the existence of long-timescale fluctuations.

Multiobjective Optimal Control of Wind Turbines: A Survey on Methods and Recommendations for the Implementation

advanced control system design for large wind turbines is becoming increasingly complex, and high-level optimization techniques are receiving particular attention as an instrument to fulfil this significant degree of design requirements. Multiobjective optimal (MOO) control, in particular, is today a popular methodology for achieving a control system that conciliates multiple design objectives that may typically be incompatible. Multiobjective optimization was a matter of theoretical study for a long time, particularly in the areas of game theory and operations research. Nevertheless, the discipline experienced remarkable progress and multiple advances over the last two decades. Thus, many high-complexity optimization algorithms are currently accessible to address current control problems in systems engineering. On the other hand, utilizing such methods is not straightforward and requires a long period of trying and searching for, among other aspects, start parameters, adequate objective functions, and the best optimization algorithm for the problem. Hence, the primary intention of this work is to investigate old and new MOO methods from the application perspective for the purpose of control system design, offering practical experience, some open topics, and design hints. A very challenging problem in the system engineering application of power systems is to dominate the dynamic behavior of very large wind turbines. For this reason, it is used as a numeric case study to complete the presentation of the paper.

Pitch Control of Three Bladed Large Wind Energy Converters—A Review

Modern multi-megawatt wind turbines are currently designed as pitch-regulated machines, i.e., machines that use the rotation of the blades (pitching) in order to adjust the aerodynamic torque, such that the power is maintained constantly throughout a wide range of wind speeds when they exceed the design value (rated wind speed). Thus, pitch control is essential for optimal performance. However, the pitching activity is not for free. It introduces vibrations to the tower and blades and generates fatigue loads. Hence, pitch control requires a compromise between wind turbine performance and safety. In the past two decades, many approaches have been proposed to achieve different objectives and to overcome the problems of a wind energy converter using pitch control. The present work summarizes control strategies for problem of wind turbines, which are solved by using different approaches of pitch control. The emphasis is placed on the bibliographic information, but the merits and demerits of the approaches are also included in the presentation of the topics. Finally, very large wind turbines have to simultaneously satisfy several control objectives. Thus, approaches like collective and individual pitch control, tower and blade damping control, and pitch actuator control must coexist in an integrated control system.

Design and simulation of an optimized small wind turbine

The volatility of fossil fuel's price, pollution, and emission associated with converting fos- sil fuel into a useful type of energy led man to search for more sustainable energy sources that are pollution-free and renewable. Today, renewable energy technologies, such as solar and large wind turbines, are developed to a stage of maturity, having the cost of produc- ing electricity dropping signi􏰀cantly in the last decade, therefore making these technologies competitive with the traditional counterpart. The cost of producing electricity through small wind turbines is still high compared to large wind turbines or photovoltaic technology. For small wind turbines to successfully compete with other technologies and contribute to the diversi􏰀cation of o􏰈-grid technology, further research is needed to reduce the levelised cost of energy (LCOE). Therefore, this study aims to reduce the levelised cost of energy (LCOE) of small wind turbines. To achieve the ob- jective, a 10 kW wind turbine operating at a site of an average wind speed of 7.5 m/s was designed, optimized, and simulated. With low LCOE in mind, the turbine components were designed as simple as possible to reduce manufacturing costs. The blades are made of uniform cross-sectional area, which made possible to use aluminum as the blade material, and the blade cross-sectional area is made out of a high lift airfoil. The hub is made of aluminum and modelled and designed as a disc with holes to bolt the blades and attach the main shaft. The mainframe is treated as a thick plate with a proper arrangement to connect the generator, the main and yaw bearings, the tail support, and any other ancillaries needed. An octal tapered tower with a height of 20 m made of steel was designed and optimized for low weight. The electrical power is to be produced by a direct drive variable speed permanent magnet synchronous generator. The control system is designed in such a way that allows the turbine to operate in maximum power e􏰊ciency for any speed below the rated speed, and to increase reliability, a sensorless control system is suggested. The research started with a broad review of the relevant literature on wind turbines in general and small wind turbines. The turbine blades design began by analysing the aero- dynamic performance of the blade. To accomplish that, XFoil was used to generate the aerodynamic parameters of the airfoil, the Blade Element Momentum (BEM) method was used to estimate the blades' aerodynamic performance, and Qblade was employed to com- pare the results, and Computational Fluid Dynamics (CFD) was used to verify the results. The preliminary design was done using standard IEC 61400-2 to obtain the load cases, and general engineering formulas, CFD and Finite Element Analysis (FEA) was used to analyse the load in the components according to IEC 61400-2, FAST-V7 was used to simulate the turbine's overall performance, standard formulas were used to evaluate the economic perfor- mance of the design, MatLab was used to perform all needed calculations. In this study, it is evident that using standard IEC 61400-2 to estimate the load, gyroscopic load components dominate the design, and the control system must be used to limit those loads. The designed turbine has relatively high e􏰊ciency and low LCOE.

Multidisciplinary Design Optimization of Extra-Large Wind Turbine Monopile in Hurricane Conditions

Offshore wind energy is developing rapidly in the United States, particularly off the East coast, which has an extensive continental shelf and where the water depths are such that wind turbines founded directly on the seabed are the most attractive. Of the various foundation configurations that are feasible, it is the monopile that is expected to be most widely used. Another feature of the US East coast is that it is hurricane-prone region, which poses some unique challenges to designers. This paper summarizes research work undertaken to investigate how the role of modern optimization techniques can contribute to the design of such installation and does not establish any realistic project related implications. The project addresses the design of a 15 MW turbine, which represents the upper limit capacity of currently available wind turbines.

Time-Dependent Upper Limits to the Performance of Large Wind Farms Due to Mesoscale Atmospheric Response

A prototype of a new physics-based wind resource assessment method is presented, which allows the prediction of upper limits to the performance of large wind farms (including the power loss due to wind farm blockage) in a site-specific and time-dependent manner. The new method combines the two-scale momentum theory with a numerical weather prediction (NWP) model to assess the “extractability” of wind, i.e., how high the wind speed at a given site can be maintained as we increase the number of turbines installed. The new method is applied to an offshore wind farm site in the North Sea to demonstrate that: (1) Only a pair of NWP simulations (one without wind farm and the other with wind farm with an arbitrary level of flow resistance) are required to predict the extractability. (2) The extractability varies significantly from time to time, which may cause more than 30% of change in the upper limit of the performance of medium-to-high-density offshore wind farms. These results suggest the importance of considering not only the natural wind speed but also its extractability in the prediction of (both long- and short-term) power production of large wind farms.

Wake properties and power output of very large wind farms for different meteorological conditions and turbine spacings: A large-eddy simulation case study for the German Bight

Germany’s expansion target for offshore wind power capacity of 40 GW by the year 2040 can only be reached if large portions of the Exclusive Economic Zone in the German Bight are equipped with wind farms. Because these wind farm clusters will be much larger than existing wind farms, it is unknown how they affect the boundary layer flow and how much power they will produce. The objective of this large-eddy-simulation study is to investigate the wake properties and the power output of very large potential wind farms in the German Bight for different turbine spacings, stabilities and boundary layer heights. The results show that very large wind farms cause flow effects that small wind farms do not. These effects include, but are not limited to, inversion layer displacement, counterclockwise flow deflection inside the boundary layer and clockwise flow deflection above the boundary layer. Wakes of very large wind farms are longer for shallower boundary layers and smaller turbine spacings, reaching values of more than 100 km. The wake in terms of turbulence intensity is approximately 20 km long, where longer wakes occur for convective boundary layers and shorter wakes for stable boundary layers. Very large wind farms in a shallow, stable boundary layer can excite gravity waves in the overlying free atmosphere, resulting in significant flow blockage. The power output of very large wind farms is higher for thicker boundary layers, because thick boundary layers contain more kinetic energy than thin boundary layers. The power density of the energy input by the geostrophic pressure gradient limits the power output of very large wind farms. Because this power density is very low (approximately 2 W m−2), the installed power density of very large wind farms should be small to achieve a good wind farm efficiency.